The present invention relates to exhaust gas sensors. More particularly, the present invention relates to an exhaust gas sensor with a crimp design.
Exhaust gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust gas sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the exhaust sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known gas partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                              where            :                    ⁢                      xe2x80x83                                                        E          =                      xe2x80x83                    ⁢                      electromotive            ⁢                          xe2x80x83                        ⁢            force                                                        R          =                      xe2x80x83                    ⁢                      universal            ⁢                          xe2x80x83                        ⁢            gas            ⁢                          xe2x80x83                        ⁢            constant                                                        F          =                      xe2x80x83                    ⁢                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                                                        T          =                      xe2x80x83                    ⁢                      absolute            ⁢                          xe2x80x83                        ⁢            temperature            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                        ref                    =                      xe2x80x83                    ⁢                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            reference            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                                =                      xe2x80x83                    ⁢                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                          xe2x80x83                        ⁢            gas                              
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
As taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., a sensor which operates in a diffusion limited current mode produces a proportional output which provides a sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions. Generally, diffusion limited current sensors have a pumping cell and a reference cell with a known internal or external oxygen partial pressure reference. A constant electromotive force, typically corresponding to the stoichiometric electromotive force, is maintained across the reference cell by pumping oxygen through the pumping cell. The magnitude and polarity of the resulting diffusion limited current is indicative of the exhaust oxygen partial pressure and, therefore, a measure of air-to-fuel ratio.
Where a gas-diffusion-limiting means is added to an oxygen pump, the pump current can be limited, and the limiting current is linearly proportional to the absolute value of the equilibrium oxygen concentration of the exhaust gas. In lean condition, the equilibrium oxygen concentration is larger than zero, which indicates a surplus of oxygen, and oxygen needs to be pumped out of the exhaust gas to create a limiting current. In the rich condition, the equilibrium oxygen concentration is smaller than zero, which indicates depletion of oxygen, and oxygen needs to be pumped into the exhaust gas to create a limiting current. Therefore, using the absolute value and the polarity of the limiting current, one can determine the air-to-fuel ratio of the exhaust gas.
However, an oxygen pump cell will not switch its current polarity automatically if both pump electrodes are exposed to the same exhaust gas. Conventional sensor technology either uses an air reference electrode as one of the pump electrodes or utilizes an air reference electrode as a third electrode to detect the lean or rich status of the exhaust gas (by emf mode) and to switch the current polarity accordingly. In this way, wide range air-to-fuel ratios of the exhaust gas can be determined.
Such conventional sensors use two types of air reference electrodes. The first type has a sizable air chamber to provide oxygen from an ambient air supply to the reference electrode (breatheable air reference). However, to avoid contamination by the exhaust gas, the air chamber requires a hermetic seal sensor package, which is expensive and is problematic in field applications. The second type is a pumped-air reference electrode. It uses a pump circuit to pump oxygen from the exhaust gas to the reference electrode. As such, it does not require a sizable air chamber connected to ambient air.
One known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. The flat plate sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of briffle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, typically have a high modulus of elasticity and cannot withstand much bending. Hence, great care and expense is expended in preventing manufacturing failures.
Accordingly, there remains a need in the art for a low cost, temperature resistant sensor package having an improved assembly and design.
The problems and disadvantages of the prior art are overcome and alleviated by the dead headed sealed air reference sensor and method preventing contamination of a sensor. The exhaust gas sensor comprises an upper shield having an upper shield first end and an upper shield second end; an inner shield positioned with a portion of the upper shield second end; a shell having a shell first end and a shell second end positioned about a portion of the inner shield, the shell first end has a projecting edge spaced apart from the inner shield, wherein a segment is formed between the projecting edge and the inner shield; a crimp formed from a bent portion of the projecting edge of the shell about a terminal end portion of the inner shield; a lower shield affixed to the shell second end; and a sensor element extending through and within the lower shield, the shell, and the upper shield.
The method of forming an exhaust gas sensor, comprises providing an upper shield having an upper shield first end and an upper shield second end; positioning an inner shield within a portion of the upper shield second end; placing a shell having a shell first end and a shell second end positioned about a portion of the inner shield, the shell first end has a projecting edge spaced apart from the inner shield, wherein a segment is formed between the projecting edge and the inner shield; forming a crimp by bending a portion of the projecting edge of the shell about a terminal end portion of the inner shield; affixing a lower shield to the shell second end; and extending a sensor element through and within the lower shield, the shell, and the upper shield.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.